Road Bike, Cycling Forums banner
1 - 4 of 4 Posts

·
Registered
Joined
·
916 Posts
Discussion Starter · #1 · (Edited)
Disc brake rotor: direction of the spokes

As I can see on the Net, this question has puzzled quite a few people before, if not virtually everyone who ever saw typical bicycle brake rotor with swept spokes.

Why are bicycle brake rotors installed so that their spokes lead into the direction of rotation, instead of trailing that direction?

Brake Disc Spokes - Why are they all Leading?!
Disc Rotors wrong way round?? - Tech Q&A - Bike Hub

Bicycle wheel rim Mode of transport Spoke Bicycle tire Bicycle part

A flat steel strip can sustain significantly greater loads under tension (before it tears) than under compression (before it buckles). A naive conclusion from this fact would be that the rotor should be installed so that the spokes trail the direction of rotation instead of leading it.

And in fact, a picture at the second link above (as well as quick search in the net for motorcycle rotors) shows that motorcycle rotors with swept spokes are usually installed in accordance with my "naive" logic: the spokes trail the direction of rotation

Tire Wheel Automotive tire Mode of transport Motorcycle

(Although motorcycle rotors are so much beefier that for them it probably matters less if at all).

The first link offers an explanation for the bicycle rotors, which allegedly originates from Hayes:

"The reason for the spoke design is that there are two sources of stresses in the rotor. The first is mechanical stresses due to torque and the second is thermal stresses within the rotor. As the braking surface heats up, it expands. The inner portion of the rotor near the hub is comparatively much cooler. With the outer braking surface expanding with higher temperature and the temperature of the center remaining largely unchanged a thermal stress is imparted on the spokes. The spoke design is specified such that the mechanical stresses and the thermal stresses occur in opposite orientations, attempting to cancel each other out and lowering the total stress in spokes as opposed to adding together. The result is the “sweeping forward” spoke pattern."

This certainly sounds as the right explanation. However, I'm having hard time figuring out the forces that are present inside the rotor subjected to thermal expansion and braking at the same time. I want to see how they "cancel each other out". Anyone has a good diagram or more detailed verbal explanation?
 

·
Registered
Joined
·
1 Posts
Disc brake rotor: direction of the spokes

As I can see on the Net, this question has puzzled quite a few people before, if not virtually everyone who ever saw typical bicycle brake rotor with swept spokes.

Why are bicycle brake rotors installed so that their spokes lead into the direction of rotation, instead of trailing that direction?

Brake Disc Spokes - Why are they all Leading?!
Disc Rotors wrong way round?? - Tech Q&A - Bike Hub

View attachment 313666

A flat steel strip can sustain significantly greater loads under tension (before it tears) than under compression (before it buckles). A naive conclusion from this fact would be that the rotor should be installed so that the spokes trail the direction of rotation instead of leading it.

And in fact, a picture at the second link above (as well as quick search in the net for motorcycle rotors) shows that motorcycle rotors with swept spokes are usually installed in accordance with my "naive" logic: the spokes trail the direction of rotation

View attachment 313665

(Although motorcycle rotors are so much beefier that for them it probably matters less if at all).

The first link offers an explanation for the bicycle rotors, which allegedly originates from Hayes:

"The reason for the spoke design is that there are two sources of stresses in the rotor. The first is mechanical stresses due to torque and the second is thermal stresses within the rotor. As the braking surface heats up, it expands. The inner portion of the rotor near the hub is comparatively much cooler. With the outer braking surface expanding with higher temperature and the temperature of the center remaining largely unchanged a thermal stress is imparted on the spokes. The spoke design is specified such that the mechanical stresses and the thermal stresses occur in opposite orientations, attempting to cancel each other out and lowering the total stress in spokes as opposed to adding together. The result is the “sweeping forward” spoke pattern."

This certainly sounds as the right explanation. However, I'm having hard time figuring out the forces that are present inside the rotor subjected to thermal expansion and braking at the same time. I want to see how they "cancel each other out". Anyone has a good diagram or more detailed verbal explanation?

This is something that has been puzzling me for a while as well. I'm not thinking failure of the disc as an issue but more noise and squeal when braking hard. If a member is in compression it wants to go everywhere but in a straight line and will buckle whereas the opposite is true when a member is in tension. This "micro buckling" could lead to brakes being noisy and not as efficient as when the spokes are in tension.

Surely motorbikes have it right.
 
1 - 4 of 4 Posts
Top